1. Field of the Invention
The present invention relates to an optical recording medium where dye is used in the recording layer, and in particular to an optical recording medium that can be recorded at high speed.
2. Description of the Related Art
Large-capacity recordable optical discs (called DVD-R discs) have come onto the market as optical recording media that are capable of digital recording. DVD-R discs are constructed with a recording layer, a reflective layer, and a protective layer being successively formed on top of one another on a light-transmitting substrate, and have been standardized as media for which recording and reproduction can be performed by a laser with a wavelength of 630 nm to 680 nm inclusive. Guide grooves that are required for recording information using a laser are formed on the light-transmitting base substrate of a DVD-R disc, with the form of such guide grooves being optimized for overall signal quality, which includes various recording and reproduction characteristics and servo signal characteristics, etc. As one example, these guide grooves are formed with a pitch around 0.6 μm to 0.7 μm. On the other hand, the recording layer is formed of a variety of recording materials, and as one example is formed of a chelate compound of a metal and a dye that is represented by the general formula (I) shown below, which is to say, a metal azo dye. 
It should be noted that in the above formula (I), A represents residue that forms, together with the carbon atom and nitrogen atom with which A is combined, a heterocyclic ring, B represents residue that forms, together with two carbon atoms with which B is combined, an aromatic ring or a heterocyclic ring, and X represents a hydroxyl group, a carboxyl group, a sulfonic acid-derivative group, or (NSO2Q)−, where Q represents an alkyl group of one to six carbon atoms that may be substituted with one or more fluorine atoms as the substituent(s).
In this case, the dye is not limited to a metal azo dye, but as shown by the characteristic W in FIG. 1, the characteristics of the dye used in a DVD-R disc (such as a metal azo dye or a cyanine dye) are such that the maximum absorption wavelength for a thin film is between 450 nm and 620 nm inclusive, which is shorter than the laser wavelength for recording and reproduction (between 630 nm and 680 nm inclusive) and the extinction coefficient for the laser wavelength for recording and reproduction is around 0.01 to 0.03. Accordingly, a recording layer that is formed using 100% of the dyes mentioned above, including the metal azo dye, will have the same characteristics, thereby ensuring a sufficiently high reflectance for the laser wavelength used for recording and reproduction. Out of such dyes, metal azo dye has an excellent light stability, so that a recording layer formed of such dye also has excellent light stability. It should be noted that for the characteristic W in FIG. 1, two maximum absorption wavelengths (around 540 nm and around 590 nm) appear in the range from 450 nm to 620 nm inclusive, but that in the present application, a group of maximum absorption wavelengths where there is little drop in the extinction coefficient between the two ends of a narrow range such as this is treated as a single maximum absorption wavelength. In this case, the maximum absorption wavelength that represents a range that is being treated as a single maximum absorption wavelength, out of the group of maximum absorption wavelengths, with the highest extinction coefficient. Accordingly, for the characteristic W in FIG. 1, since the wavelength of around 540 nm has a higher extinction coefficient than the wavelength of around 590 nm, around 540 nm is set as the maximum absorption wavelength. This is also the case for the rest of this specification.
An optical disc that is constructed of a recording layer composed of 100% of the dyes mentioned above, such as metal azo dye, has the following drawback that needs to be improved. According to the fundamental specification for DVD-R discs, the linear velocity is set at 3.49 m/s (single speed) and such optical recording media are designed so that recording can be performed at this speed. Optical recording media that can be recorded at a faster speed, such as double speed, have also been verified. However, recently there have been demands for DVD-R discs that can be recorded at higher linear velocities of 10 m/s and above (such as quad speed). Experiments conducted by the inventors of the present invention have confirmed that when recording is performed using a recording laser with a conventional output, the recording layer is not sufficiently sensitive, so that recording on an optical recording medium is problematic at a linear velocity of 10 m/s or above. Experiments have also confirmed that it is possible to perform recording at quad speed using methods such as raising the output of the recording laser to compensate for the insufficient sensitivity of the recording layer or changing the composition of the recording layer to raise the sensitivity. However, there are limits on the output of a recording laser, and in view of factors such as the durability of the laser, it is preferable for the output of the recording laser to be a predetermined level or below. Accordingly, methods that raise the output of the recording laser to compensate for the insufficient sensitivity of the recording layer are not realistic. On the other hand, with methods that make significant changes to the composition of the recording layer, it is difficult to maintain the electrical characteristics (excellent light stability and favorable reflectance) of the recording layer that is composed of the DVD-R dyes described above, and in many cases such changes cause problems, such as an increase in the error rate or a decrease in the reflectance. Accordingly, one issue for current optical recording media is that it is difficult to perform recording at high speed while maintaining the excellent light stability and favorable reflectance.